Background: Blowfly larvae and pupae collected during post-mortem forensic investigation can be analysed to detect drugs or toxins in a dead body. Drug or toxin-contaminated tissues of human remains consumed by blowfly larvae are stored in the crop even after a body has decomposed to the point where no fluid or tissues are retrievable. Therefore, blowfly larva and pupa are favourable specimens under conditions in which host tissues or blood samples are not available. Methodology: We carried out a study to detect heroin metabolites in Lucilia cuprina (Weidemann, 1830) immatures fed on different concentrations of heroin-treated meat: 500, 1000, 2500, 5000, and 10,000 ng/μl. Heroin metabolites detection was performed by a gas chromatography mass spectrophotometry (GCMS) analysis.Results: This study indicates that L. cuprina can be utilized in a toxicological analysis to detect heroin in a host. However, the expected complete metabolites of heroin were not detected in the first instar larvae and pupa, while the second and third instar larvae contributed to a complete heroin metabolite which was morphine. This was subject to the heroin concentration given. Morphine was detected in the third instar larvae treated with 5000 and 10,000 ng/μl heroin, while in the second instar, it was detected only at 10,000 ng/μl. Heroin metabolites detected in the second and third instar larvae proves that biochemical conversions occur in the fly larvae. Conclusion:The absence of heroin metabolites in the first instar larvae and pupa does not necessarily mean that the drug was not present in the host. Therefore, the limitations of heroin detection capability during these stages should be always kept in mind, as this could contribute to a false-negative result in a forensic analysis. If all stages are available during sample collection, it is crucial to prioritize the third instar larvae in a toxicological analysis.